GLT Design and Structural Performance

Glued laminated timber (GLT) is designed using characteristic material properties that reflect its engineered, laminated nature, rather than the properties of individual sawn boards. These properties form the basis of structural design under AS 1720.1 and are typically provided through a combination of standardised grades and supplier technical data.

Characteristic Strength Properties

The primary strength properties used in GLT design include:

• Bending strength, governing the capacity of beams and lintels
• Tension parallel to grain, relevant for bottom fibres in bending and for axially loaded members
• Compression parallel to grain, governing columns and compression zones in beams
• Shear strength, generally governing deep or heavily loaded members

In GLT, laminations are arranged so that higher-grade material is typically placed in the outer zones of the section, where stresses are greatest. This results in more efficient use of timber resources and improved structural reliability compared to solid sawn members of equivalent size.

Stiffness and Modulus of Elasticity

Stiffness in GLT members is characterised by the modulus of elasticity (MOE), which governs deflection and vibration behaviour. Due to the lamination process and grading of individual laminates, GLT exhibits:

• More consistent stiffness properties than solid sawn timber
• Reduced sensitivity to local defects such as knots
• Predictable deflection behaviour under service loads

Design values for MOE are typically provided as characteristic or mean values and should be applied in accordance with AS 1720.1 requirements for serviceability limit state checks.

Influence of Lamination on Structural Performance

The laminated construction of GLT provides several structural benefits:

• Improved strength uniformity along the member length
• Reduced likelihood of critical defects governing performance
• Enhanced reliability for long-span applications

However, designers should be aware that GLT remains an orthotropic material, with strength and stiffness predominantly aligned with the grain direction. Design assumptions regarding load paths, bearing, and restraint must therefore remain consistent with timber behaviour.

Variability and Reliability

Compared to solid sawn timber, GLT exhibits lower variability in strength and stiffness, which underpins its suitability for engineered applications. This improved reliability is reflected in:

• Higher characteristic design values for equivalent section sizes
• Greater confidence in span tables and pre-engineered solutions
• More predictable serviceability performance over time

Nevertheless, the performance of GLT members is dependent on correct specification of grade, service class, load duration, and environmental conditions, all of which must be clearly defined in the structural design documentation.

The structural capacity of glued laminated timber (GLT) members is strongly influenced by the available section sizes and the way in which those sections are deployed within the structural system. Compared to solid sawn timber, GLT allows larger, deeper, and more consistent sections to be manufactured, enabling efficient load-bearing members and longer clear spans.

Typical Section Sizes

GLT is commonly supplied in a range of standard widths and depths, with member length governed primarily by manufacturing and transport constraints. Typical characteristics include:

• Standard widths aligned with lamination thicknesses and manufacturing formats
• Variable depths, allowing structural capacity to be tuned to span and loading demands
• Longer member lengths than are generally achievable with solid timber

Figure 1: Typical Unit Planks that make up GLT elements (Note: Increments may vary depending on timber species and sawmill supply chains)

Figure 2: Element Size Limits (Note: Increments may vary depending on timber species and sawmill supply chains)

While standard sizes are widely available, many suppliers also offer bespoke section sizes for project-specific applications, subject to lead times and minimum order quantities.

Relationship Between Section Size and Capacity

Load-bearing capacity in GLT members increases with section depth and width, but not uniformly across all actions:

• Bending capacity is highly sensitive to section depth
• Shear capacity becomes critical in deeper or heavily loaded members
• Axial capacity in compression or tension is influenced by both cross-sectional area and member slenderness

The ability to increase depth efficiently makes GLT particularly well suited to beams, lintels, and transfer elements where bending governs design.

Slenderness and Stability Considerations

For compression members and members subject to combined actions, slenderness effects must be considered. GLT columns and compression members require checks for:

• Buckling about the major and minor axes
• Effective length determined by end restraint conditions
• Interaction between axial load and bending

These considerations are addressed using the design methods prescribed in AS 1720.1 and may govern section selection for tall or lightly restrained members.

Practical Limits and Design Implications

While GLT enables larger sections than solid timber, practical limits still apply:

• Transport and handling constraints can govern maximum section size
• Connection detailing may limit the effective capacity of large members
• Architectural integration may constrain member depth

Designers should therefore consider section size selection in conjunction with connection design, construction methodology, and architectural requirements, rather than assessing member capacity in isolation.

Overall, the availability of a wide range of GLT section sizes allows designers to balance structural efficiency, constructability, and aesthetics, provided that design assumptions and load-bearing requirements are clearly defined from the outset.

Structural design of glued laminated timber (GLT) members in Australia is carried out using the limit state design framework set out in AS 1720.1, as referenced by NCC 2022. Within this framework, GLT is treated as an engineered timber product with defined characteristic properties, modification factors, and design actions.

Application of Limit State Design Principles

GLT members are assessed against:

• Ultimate Limit States (ULS), ensuring adequate strength and stability under factored loads
• Serviceability Limit States (SLS), ensuring acceptable performance under normal use

Design actions are derived in accordance with AS/NZS 1170, while material capacities and design methods are taken from AS 1720.1.

Figure 3: Visual representation of the aims of Limit State Design

Ultimate Limit State Considerations

At ULS, GLT members must be checked for all relevant strength limit states, including:

• Bending capacity, typically governing beams and lintels
• Shear capacity, which may govern deep or short-span members
• Axial capacity in tension or compression
• Combined actions, where bending interacts with axial load

Modification factors related to load duration, moisture, temperature, and service class are applied in accordance with AS 1720.1. These factors are particularly important for GLT members exposed to varying environmental conditions.

Serviceability Limit State Considerations

Serviceability performance often governs the design of GLT members, particularly in long-span applications. SLS checks include:

• Deflection under service loads, including long-term effects
• Vibration performance, where applicable to floors or occupied spaces
• Visual performance, especially for exposed members

Creep effects must be accounted for in accordance with AS 1720.1, with long-term deflection often forming a significant portion of total movement.

Design Assumptions Specific to GLT

While the design process for GLT aligns closely with that of other timber products, some considerations are specific to glued laminated members:

• Assumption of composite action across laminations
• Reliance on manufacturing quality control to achieve design properties
• Use of supplier-provided characteristic values consistent with AS 1720.1

Designers should ensure that the GLT product specified is certified and graded to the design values assumed, and that any deviations from standard grades or properties are explicitly addressed in the design documentation.

By applying the limit state design provisions of AS 1720.1 rigorously, GLT members can be designed to achieve reliable strength and serviceability performance across a wide range of structural applications.

The structural performance of glued laminated timber (GLT) members is governed by their behaviour in bending, tension, and compression, both as individual actions and in combination. The laminated construction of GLT leads to more predictable stress distribution and strength performance than solid sawn timber, particularly for members subjected to bending.

Bending Behaviour

Bending is the most common governing action for GLT beams and lintels. In bending:

• Tensile stresses develop in the outer fibres on the tension side of the member
• Compressive stresses develop in the outer fibres on the compression side
• Stresses reduce towards the neutral axis

GLT is typically manufactured with higher-grade laminations placed in the outer zones, where bending stresses are greatest. This configuration improves bending efficiency and reduces the likelihood of local defects governing capacity.

Bending strength checks are carried out in accordance with AS 1720.1, with modification factors applied for load duration, service class, and environmental conditions.

Figure 4: Diagram of the Structural Effects of a GLT Element in Bending

Tension Parallel to Grain

Tension parallel to grain is critical for:

• Bottom fibres of beams under bending
• Members subject to axial tension

Although GLT improves reliability compared to solid timber, tension capacity remains sensitive to defects, adhesive performance, and stress concentrations. Designers should:

• Avoid detailing that introduces high tensile stresses perpendicular to the grain
• Pay particular attention to connection zones where tension forces are transferred

In many cases, tension parallel to grain governs the design of highly stressed GLT members.

Compression Parallel to Grain

Compression parallel to grain governs the design of:

• Columns
• Compression zones in beams
• Members subject to combined bending and axial load

For compression members, member stability is often as important as material strength. Slenderness effects, effective length, and end restraint conditions must be assessed in accordance with AS 1720.1. In short, stocky members, material strength may govern, while in slender members, buckling effects typically control.

Combined Actions and Interaction Effects

GLT members frequently experience combined bending and axial actions, particularly in portal frames, arches, and heavily loaded beams. AS 1720.1 provides interaction equations to assess combined effects.

Designers should ensure that:

• All relevant load combinations are assessed
• Interaction effects are not overlooked in preliminary sizing
• Governing load cases are clearly documented

Understanding how bending, tension, and compression interact is critical to achieving efficient and robust GLT designs without relying on excessive conservatism.

Serviceability performance is often the governing design criterion for glued laminated timber (GLT) members, particularly in long-span or architecturally exposed applications. Deflection control is therefore a critical aspect of GLT design and must be addressed alongside strength checks.

Short-Term and Long-Term Deflection

Total deflection in GLT members comprises:

• Short-term deflection due to immediate elastic response under applied loads
• Long-term deflection resulting from creep under sustained loading

AS 1720.1 requires both components to be considered in serviceability limit state (SLS) checks. For GLT, long-term deflection can represent a significant proportion of total movement, especially in roof and floor systems subjected to permanent loads.

Creep Behaviour in GLT

While GLT exhibits more consistent creep behaviour than solid sawn timber, it remains sensitive to:

• Load duration
• Service class and moisture conditions
• Stress level relative to capacity

Creep factors prescribed in AS 1720.1 should be applied conservatively, particularly where members are exposed to elevated humidity or fluctuating environmental conditions.

Serviceability Criteria and Limits

Deflection limits for GLT members are typically governed by:

• Functional performance of the structure
• Compatibility with finishes, cladding, and partitions
• Visual acceptability for exposed members

Common deflection limits are often expressed as a proportion of span, but designers should confirm project-specific requirements rather than relying solely on generic values.

Design Implications for Long-Span Members

In long-span applications, deflection control can dictate section selection more strongly than strength. Designers should consider:

• Increasing section depth to improve stiffness efficiency
• Introducing intermediate supports where feasible
• Coordinating structural depth with architectural intent

Early consideration of deflection behaviour helps avoid costly redesigns and ensures that GLT members perform satisfactorily throughout their service life.

Overall, effective deflection and serviceability control is essential to realising the structural and architectural benefits of GLT, particularly where members are visible and form a key part of the building expression.

Glued laminated timber (GLT) members are specified using both structural grades, which govern load-bearing capacity, and visual or appearance grades, which govern aesthetic quality. These two grading systems serve different purposes and should not be treated as interchangeable in design or documentation.

Structural Grading

Structural grades define the characteristic strength and stiffness properties used in engineering design. For GLT, these grades are based on:

• The grading of individual laminations
• The layup configuration of laminations within the member
• Manufacturing quality control and certification

Structural grades form the basis of design values used in AS 1720.1 checks and must be clearly specified by the designer to ensure that the assumed design capacities are achieved.

Visual and Appearance Grading

Visual or appearance grades relate to the surface finish and visible characteristics of the GLT member, such as:

• Knot size and distribution
• Colour variation
• Surface checks and minor imperfections

Appearance grades are particularly relevant where GLT is exposed as a finished architectural element. However, they do not, on their own, define structural performance.

Relationship Between Visual and Structural Performance

While appearance grades may impose additional selection or finishing requirements, they should not be assumed to provide enhanced structural capacity. A visually high-quality member may still have the same structural grade as a less visually refined product.

Designers should:

• Specify structural grade as the primary performance requirement
• Nominate appearance grade separately where exposed members are intended
• Avoid using appearance terminology as a proxy for structural capacity

Specification and Documentation Considerations

Clear separation of structural and visual requirements in project documentation helps avoid:

• Misinterpretation during procurement
• Unintended substitution of products
• Disputes between design and construction teams

By explicitly defining both structural and appearance grades, designers can ensure that GLT members achieve the required performance while meeting architectural expectations.

Span tables are commonly used to support preliminary sizing and specification of glued laminated timber (GLT) members. When used appropriately, they provide a practical shortcut for common design scenarios. However, their assumptions and limitations must be clearly understood to avoid misuse.

Purpose and Appropriate Use of Span Tables

GLT span tables are typically intended for:

• Early-stage design and feasibility studies
• Standardised residential and low-rise commercial applications
• Common loading and support conditions

They are not a substitute for project-specific engineering design where loads, spans, or boundary conditions fall outside typical assumptions.

Underlying Assumptions

Span tables are developed based on a defined set of assumptions, which generally include:

• Uniformly distributed loads of a specified magnitude
• Simply supported boundary conditions
• Prescribed deflection limits at serviceability limit state
• A defined service class and load duration
• Specific GLT structural grades

Designers must confirm that the project conditions align with these assumptions before relying on tabulated spans.

Strength vs Serviceability Control

In many GLT span tables, serviceability criteria govern the tabulated spans, rather than ultimate strength. This reflects the importance of deflection control in timber structures, particularly for longer spans.

As a result:

• Members may have reserve strength beyond the tabulated span
• Increasing span beyond table limits without engineering assessment is not permitted
• Changes in deflection criteria can significantly alter allowable spans

Limitations of Span Tables

Span tables have inherent limitations, including:

• Inability to account for point loads or non-uniform loading
• Limited treatment of combined bending and axial actions
• Assumed connection behaviour and bearing conditions
• Conservative simplifications to cover a range of applications

Where these limitations are present, full structural design in accordance with AS 1720.1 is required.

Relationship to Engineered Design

Span tables should be viewed as a screening and guidance tool, rather than a definitive design solution. For non-standard conditions, long spans, heavily loaded members, or architecturally exposed structures, engineered design provides:

• More efficient member sizing
• Explicit treatment of governing load cases
• Clear alignment between member capacity and connection design

Used appropriately, GLT span tables can streamline design decisions, provided their assumptions are respected and their limits are not exceeded.

The structural performance of glued laminated timber (GLT) members cannot be considered in isolation from their connections. In many cases, connection design governs the overall system capacity, stiffness, and serviceability behaviour, particularly where forces are transferred through discrete fasteners or bearing interfaces.

Role of Connections in Structural Performance

Connections are responsible for:

• Transferring forces between members and to supporting elements
• Providing restraint that influences effective length and stability
• Controlling rotation and slip at supports

In GLT structures, connection behaviour can significantly affect both ultimate strength and deflection performance, even where the member itself has adequate capacity.

Connection Stiffness and Member Behaviour

The stiffness of connections influences:

• Distribution of internal forces
• Apparent span and boundary conditions
• Overall system deflection

Assumptions of simple support or full fixity used in member design must be consistent with the actual connection detailing. Overly stiff or overly flexible connections can lead to performance that differs materially from design assumptions.

Local Effects and Stress Concentrations

Connection regions often introduce:

• Local bearing stresses perpendicular to grain
• Stress concentrations around fasteners
• Reduced effective section due to notching or drilling

These local effects can govern design and must be checked in accordance with AS 1720.1. Careful detailing is particularly important in highly stressed GLT members, where connection zones may become the critical design location.

Coordination Between Member and Connection Design

Effective GLT design requires coordination between:

• Member sizing and strength checks
• Connection capacity and stiffness
• Constructability and installation sequencing

Designers should ensure that:

• Connection capacities meet or exceed member demands
• Detailing assumptions used in member design are achievable in practice
• Supplier connection systems are compatible with the specified GLT grade

By considering member and connection behaviour together, designers can avoid unintended capacity reductions and achieve more reliable and efficient GLT structural systems.

Glued laminated timber (GLT) sits within a broader family of structural timber products, each with distinct design characteristics. Understanding how GLT compares with solid sawn timber and laminated veneer lumber (LVL) assists designers in selecting the most appropriate product for a given application and in applying design assumptions correctly.

GLT Compared to Solid Sawn Timber

Compared to solid sawn timber, GLT offers:

• Greater section sizes, enabling longer spans and higher load capacities
• Improved strength and stiffness consistency, due to grading and lamination
• Reduced influence of natural defects, as critical laminations are selected and positioned deliberately

Solid sawn timber remains well suited to smaller spans and lightly loaded applications, but is more constrained by available sizes and greater material variability. For larger structural members, GLT generally provides more reliable and predictable performance.

GLT Compared to LVL

When compared with LVL, GLT exhibits different structural characteristics:

• GLT typically offers greater section depth flexibility, particularly for beams and curved members
• LVL generally provides higher stiffness and strength per unit depth, reflecting its veneer-based manufacture
• GLT is often preferred for architecturally exposed elements, due to its visual qualities

From a design perspective, LVL may be more efficient for shallow floor beams or highly stressed members, while GLT is often favoured for long spans, transfer beams, columns, and expressive structural forms.

Design and Specification Implications

Key considerations when selecting between GLT, solid timber, and LVL include:

• Required span and load capacity
• Governing serviceability criteria
• Availability of section sizes and lengths
• Connection detailing requirements
• Architectural and finish expectations

No single product is universally superior. Effective timber design relies on selecting the product whose structural behaviour, availability, and constructability best align with the project requirements.

By understanding the relative strengths and limitations of GLT in comparison to other timber products, designers can make informed decisions and avoid misapplication of design assumptions across different material systems.

Designing with glued laminated timber (GLT) requires careful attention to both structural performance and practical implementation. While GLT offers significant advantages in strength consistency, section size availability, and architectural expression, its performance is dependent on appropriate application of design principles and clear specification.

Key considerations for designers include:

• Serviceability often governs design, particularly for long-span or exposed members where deflection and creep control are critical
• Structural grade, not appearance grade, determines capacity, and must be clearly specified in design documentation
• Span tables are guidance tools, suitable only where project conditions align with their assumptions
• Connection design can govern overall performance, and must be coordinated with member sizing and design assumptions
• Environmental conditions and service class matter, influencing both strength modification factors and long-term behaviour
• Product selection should be application-specific, balancing efficiency, availability, constructability, and architectural intent

Clear documentation of assumptions, load cases, and performance criteria is essential to avoid misinterpretation during procurement and construction.

When designed and detailed in accordance with AS 1720.1 and aligned with NCC 2022 requirements, GLT provides a robust and versatile structural solution across a wide range of building types.